But its so simple...
To me, it seems that the Universes that are so close in time and space are also separated by a "vast gulf" (one that the Red Tornado was forced to span)...since each had three dimensions as separate realities, ech should have their own 5th Dimension...
Not that Criadoman's explanation isn't interesting...though it might be argued that "magic" worked differently on Earth S than on Earth 1...
Actually, I would accept any good writer's explanation...